Selecting description of 12 hour time period SQL Server 2008 - sql

Question

I am writing a query for a report in which I need to select if the time period falls into the "Day" or "Night". The table I wish to select from looks like this:
DAYNIGHT_TABLE:
Description | StartTime | EndTime
Night | 18:00:00.0000000 | 06:00:00.0000000
Day | 06:00:00.0000000 | 18:00:00.0000000
For different customers this may be broken up further into smaller time slots but as a starting point I just need to select Day or Night based on a datetime that I retrieve from another table.
If I do something like this then I only get day:
SELECT Description
FROM Table s
WHERE convert(TIME,myTable.MyDateTime) >= s.StartTime
AND convert(TIME,myTable.MyDateTime) < s.StartTime
I feel like I am missing something obvious here but how can I retrieve the Day/Night Description from DAYNIGHT_TABLE based on the known time from myTable.MyDateTime?

Answer 1

If you can't touch the DAYNIGHT_TABLE table at all, read below.
Since all you need is a label "Day" or "Night" I would recommend to alter your DAYNIGHT_TABLE table and include one more row into it, so that no range of times in any row goes across the midnight. It makes query much simpler and efficient:
DAYNIGHT_TABLE:
Description | StartTime | EndTime
Night | 18:00:00.0000000 | 00:00:00.0000000
Night | 00:00:00.0000000 | 06:00:00.0000000
Day | 06:00:00.0000000 | 18:00:00.0000000
Also, make sure that you consistently assume which end of the interval is inclusive and which is exclusive. I'll assume that it is [StartTime; EndTime), i.e. StartTime inclusive and EndTime exclusive.
Have an index on StartTime DESC with Description as included column. It is not really needed and with so few rows in DAYNIGHT_TABLE you would likely not notice any difference, but still.
Now, if you have a table myTable with column MyDateTime and you want to get correct matching description from DAYNIGHT_TABLE, use something like this:
SELECT
myTable.MyDateTime
,CA.Description
FROM
myTable
CROSS APPLY
(
SELECT TOP(1) DAYNIGHT_TABLE.Description
FROM DAYNIGHT_TABLE
WHERE DAYNIGHT_TABLE.StartTime <= CAST(myTable.MyDateTime AS time)
ORDER BY DAYNIGHT_TABLE.StartTime DESC
) AS CA
For every row in myTable CROSS APPLY would find one matching Description.
Make sure that intervals in DAYNIGHT_TABLE cover all 24 hours without gaps.
Notice, that query doesn't use EndTime at all, because it assumes that intervals in the table cover full 24 hours. So, you can remove this column from the table and it will look like this:
Description | StartTime
Night | 00:00:00.0000000
Day | 06:00:00.0000000
Night | 18:00:00.0000000
With such table that has these two columns the only index that is needed is just a unique clustered primary key on StartTime.
If you can't touch the DAYNIGHT_TABLE table at all, but if you still can guarantee that intervals in the table (a) don't overlap, (b) don't have gaps, (c) cover full 24 hours, then there will be one extra step in the query.
If all conditions outlined above hold true, then there can be at most only one row that spans across midnight, which means that its StartTime is greater than its EndTime. This is how we can find it and take care of it. Again, I assume that StartTime is inclusive and EndTime is exclusive.
DECLARE #DAYNIGHT_TABLE TABLE(Description varchar(50), StartTime time, EndTime time);
INSERT INTO #DAYNIGHT_TABLE(Description, StartTime, EndTime) VALUES
('Night', '18:00:00', '06:00:00'),
('Day', '06:00:00', '18:00:00');
DECLARE #myTable TABLE(MyDateTime datetime);
INSERT INTO #myTable (MyDateTime) VALUES
('2015-01-01 00:00:00'),
('2015-01-01 02:02:02'),
('2015-01-01 06:00:00'),
('2015-01-01 12:02:02'),
('2015-01-01 18:00:00'),
('2015-01-01 22:02:02');
WITH
CTE_TimeIntervals
AS
(
SELECT
Description
,StartTime
FROM #DAYNIGHT_TABLE AS DAYNIGHT_TABLE
UNION ALL
SELECT
Description
,CAST('00:00:00' AS time) AS StartTime
FROM #DAYNIGHT_TABLE AS DAYNIGHT_TABLE
WHERE StartTime >= EndTime
)
SELECT
myTable.MyDateTime
,CA.Description
FROM
#myTable AS myTable
CROSS APPLY
(
SELECT TOP(1) CTE_TimeIntervals.Description
FROM CTE_TimeIntervals
WHERE CTE_TimeIntervals.StartTime <= CAST(myTable.MyDateTime AS time)
ORDER BY CTE_TimeIntervals.StartTime DESC
) AS CA
Result set
MyDateTime Description
2015-01-01 00:00:00.000 Night
2015-01-01 02:02:02.000 Night
2015-01-01 06:00:00.000 Day
2015-01-01 12:02:02.000 Day
2015-01-01 18:00:00.000 Night
2015-01-01 22:02:02.000 Night

Answer 2

You need somthing like this, test it with different values:
http://www.sqlfiddle.com/#!3/d6328/1/0
select s1.myDate, t.Descr
from s s1
inner join DAYNIGHT_TABLE t on datepart(hour, s1.myDate) >= datepart(hour, t.StartTime)
AND datepart(hour, s1.myDate) < datepart(hour, t.EndTime)
OR ((t.EndTime < t.StartTime) AND ( datepart(hour, s1.myDate) >= datepart(hour, t.StartTime)) AND datepart(hour, s1.mydate) < (datepart(hour, t.StartTime) + datepart(hour, t.EndTime)))